Roy knows whereof he speaks... Just for fun let me stir the pot here
and introduce a different way of looking at the mobile antenna
system...

The mobile antenna can be viewed as an off center fed dipole (OCF)...
The mast and top hat form one side of the dipole and the vehicle (the
so called ground) the other side... The problems I see - besides the
low radiation resistance of the electrically short mast and "ground" -
is that the L/D ratio of the two parts of the antenna are vastly
different...
The mast portion has an LD ratio of ~20 picked out of thin air, I am
sure someone will correct me
The "ground" has a ratio of less than 1.0
This massive offset in L/D between the two parts of the antenna system
results in massive skewing of the capacitative coupling to actual
ground on each half of the antenna...

What I propose is to not connect the RF of the antenna system directly
to the vehicle chassis... Use an insulated mast as usual, but do not
connect the coax braid to the vehicle... Instead, use the braid of RG8
as a radial/counterpoise to the mast... Run the coax underneath the
vehicle and wind it around the perimeter leaving it a shorter than an
electrical quarter wave... Leave it insulated from the vehicle... Seal
the ends against water and salt intrusion... Use a coil at the feed
point to establish resonance on this counterpoise... Compare the field
strength to the conventional method...

denny / k8do